Phase noise in RF systems is often the result of short term deterministic and random frequency fluctuations about a nominal carrier frequency. These fluctuations typically have a duration of less than a few seconds and are usually represented and viewed in the frequency domain. As the performance of phase noise measurement systems improves towards the thermal noise floor dictated by kTB noise, or −174 dBm/Hz, there is a urgent need for corresponding RF sources with lower phase noise to calibrate these systems.
Effinger et al., U.S. Pat. No. 4,516,085, relates to a microwave frequency synthesizer using a plurality of switchable low noise oscillators. The frequency synthesizer includes a plurality of oscillators 20, a plurality of switches 21 one associated with each of the oscillators 20, and a high speed isolation switch 22 that cooperates with the switches 21 to select one of the oscillators 20. Another switch 23 is operative to select a multiplication factor via a multiplier 30 and filter 31, leading to another switch 32. From switch 32, an isolator 33 provides impedance matching to a mixer 41. The other input to the mixer 41 comes from one of a plurality of other oscillators in a master oscillator bank 10 through a switch 11, filter means 12 and isolator 13.
A significant problem with the frequency synthesizer of Effinger et al. is that the large number of electronic components between the oscillators 10, 20 contribute phase noise to the signal, in spite of the presence of the filter means 12 and filters 31.
Bernhard et al., U.S. Pat. No. 4,206,421, relates to an arrangement for synchronizing a free-swinging oscillator system to a reference signal of a substantially lower frequency. A regulating circuit 6 connects to the oscillator 4 to control the frequency thereof. A quartz oscillator 1 produces a reference signal that is subsequently connected to a frequency multiplier and/or frequency divider 2, an amplifier 3 and a bandpass filter 7. An output signal from the filter 7 is fed to a harmonics mixer 4′ connected to the free-swinging oscillator 4 and which produces a pattern of harmonics. A selective amplifier 5 feeds an intermediate frequency obtained from the harmonics mixer 4′ to the regulator circuit 6. A filter 8 connected to the quartz oscillator 1 produces harmonics which are also coupled to the regulating circuit 6. By phase or frequency comparison, the regulating circuit 6 adjusts the running frequency of the free-swinging oscillator. In essence, Bernhard et al. describes a classic phase lock loop (PLL) circuit which synchronizes (locks) one oscillator to a reference oscillator.
These prior art arrangements do not include a streamlined arrangement of electronic components that enable a signal with very low noise to be generated, i.e., a very pure signal. Rather, for example, the presence of a significant number of switching components in Effinger et al. provide the exact opposite effect in that they introduce noise into the signals being processed.